President Barack Obama will tell the American people in a televised address tonight from the US Military Academy at West Point that the goal of sending 30,000 additional soldiers to Afghanistan is to "prevent the Taliban from overthrowing the Afghan government," a senior administration official said Tuesday in a background briefing with reporters.

"The concept that [Obama will] describe is to surge American forces to do several things: first, to reverse the Taliban's momentum, which has been building steadily over the last three or four years; to secure key population centers, especially in the south and the east; to train Afghan forces, and then as quickly as possible transfer responsibility to a capable Afghan partner."

The senior administration official said increasing US presence in the region, an effort opposed by a majority of the American people as well as Democrats and some Republican lawmakers, will also serve to help Pakistan "stabilize their state."

"That second part, stabilizing Pakistan, really has three dimensions: a political dimension, an economic dimension and a security dimension," the senior administration official said. "The Pakistanis require help across all three of these aspects, in particular on the security front where they face internal extremists, the Pakistani Taliban, if you will, who actually threaten their state. But also on the political and economic front, the Pakistanis require our assistance, and our long-term aim with Pakistan is to establish and then sustain a strategic partnership, which helps them bring stability to their state; in turn, to the region."

In his speech this evening, Obama will announce that he intends to deploy another 30,000 US soldiers to Afghanistan by next summer in an effort to "degrade the Taliban in order to provide time and space to develop Afghan capacity."

"They also want to degrade the Taliban for a second purpose, and that is so that as we begin to hand off responsibility to the Afghan army and police, those emerging security forces are able to handle the Taliban because it's at a diminished strength," the senior administration official said, adding that Obama "will also announce that this surge, if you will, will be for a defined period of time.

"The other key task for the military, this additional 30,000 over the coming months, is to train and partner with the Afghan security forces to accelerate their development. The broad aim here is to open a new window of opportunity for Afghanistan and to create conditions to begin to transfer to Afghan responsibility by a date which the President will specify in his speech."

The senior administration official added that Obama will also discuss "a date by which he has given the mission that we will begin to transfer our lead responsibility -- that is, the US and NATO lead responsibilities from that operation -- to Afghan counterparts. He will not, however, tonight specify the end of that transition process, nor will he specify the pace at which it will proceed. Those variables -- pace and end -- will be dictated by conditions on the ground."

The President will make a strong point tonight that this is not an open-ended commitment. And the idea here is that all of us are -- have to have a sense of urgency about this opportunity in the coming months to shift the momentum in Afghanistan. That sense of urgency has to be imparted first to our own government, both on the military side and the civilian side, but equally important to our NATO counterparts, and perhaps more important of all to our Afghan partners and our Pakistani partners.

While we do not intend -- and the President will make this very clear tonight -- to commit American combat forces indefinitely to Afghanistan, we do reaffirm our long-term strategic partnership with Afghanistan, but not at anything like 100,000 U.S. troops in their country.

Let me just quickly remind that this is not a U.S. mission alone. There are about 40,000 other ISAF or NATO forces in Afghanistan, above and beyond the 68,000 Americans who are there today. Those come from 44 other countries, so it's a pretty broad-based coalition. And we believe that by the time NATO holds its ministerial meetings at the end of this week, that the NATO Secretary General will have positive indicators that those 44 additional countries will also step forth with more contributions of troops.

Before we go to your questions, let me just remind that the military side of this equation is only one side of the coin. The civilian side is equally important. The President will announce some refinements in our approach on the military side. He will talk about how we're sending additional civilian experts to Afghanistan to team up, to partner with our military units. He'll emphasize that our approach has to be well beyond the Afghan capital of Kabul and the central government ministries, and has to reach out, in a bottom's up approach, out into the provinces and districts so that we generate a bottom-up dynamic in terms of meeting the sharp timelines that he has put us on.

And finally I'd just mention that we have established -- and the President will announce tonight -- that our top development priority in Afghanistan will from here forward be agriculture, which is very much sort of swimming with the stream and with the traditions of the agriculturally-based Afghan economy, and also offers the best promise for quickest results in terms of our economic assistance.